Humidity sensors which utilize a metal oxide ceramic semiconductor element, in which the ion conductivity changes upon absorption of water or water vapor, are known. One such structure is described in European Patent No. EP-A-33 521. The conductivity of the sensor is measured in an electrical circuit. Polarization effects at the electrodes of the sensors should be avoided and, thus, measuring the conductivity of the sensor with direct current is not possible if the application of direct current extends for an appreciable period of time. The European Patent 33 521 describes a sensor which is serially connected with an alternating current source and electrical resistor. The alternating voltage measured across the resistor will be representative of relative humidity. The alternating voltage measured signal is converted into a d-c signal in a rectifier. The system requires an alternating current generator or a bipolar supply source. Operating such a sensor from a d-c source, for example a vehicle battery, thus is not possible without additional circuitry. The disadvantage of requiring a bipolar supply source is particularly serious in battery-operated circuits since an electrical center or neutral terminal can be obtained only with additional circuit components, which introduce losses and further introduce complexity and are sources of malfunction.
British Patent No. 1,422,182 describes a humidity sensor which is made of Cr.sub.2 O.sub.3 and V.sub.2 O.sub.5. The proportion--by weight--of the components is 1:1. The sensor is constructed in form of a small globule or pearl, and sintered at 1550.degree. C. When finished, the resistance is comparatively. Constructing such a sensor is complex, and the high internal resistance reduces the measuring sensitivity thereof. Further, it has been found in actual operation that such sensors are sensitive to interference or disturbance pulses. Disturbance pulses and switching pulses and variations in supply circuit voltage, including peaks which occur upon sudden switching ON and switching OFF of loads occur, however, frequently in automotive vehicles.